1. Field of the Invention
The present invention relates to a multi-range, belt-type, continuously variable transmission. More particularly, the invention relates to a multi-range, belt-type, continuously variable transmission suitable for application in automotive drivetrains.
2. Description of the Related Art
Over the years, many vehicle transmission systems have been designed to utilize a belt-type continuously variable transmission (belt-type CVT) for multiplying engine torque to accelerate a vehicle from a standing start to a maximum cruising speed. Belt-type CVTs originated with the segmented metal compression belt invention conceived years ago by Dr. Hub Van Doorne of the Netherlands and are currently being applied in light-weight automotive applications. However, most of the belt-type CVTs are single-ratio range transmissions, which are adequate only for vehicles of about 100 kW capacity.
Moreover, belt-type CVTs have other limitations that require added complexity for their adaptation in automotive transmission applications. First, belt-type CVTs cannot provide a transmission neutral, i.e., 0:1 speed ratio, due to their minimum sheave requirements. Therefore, a disconnect device, such as a clutch, must be installed between the engine and the transmission in order to achieve a neutral state. The clutch is then gradually engaged to smoothly accelerate the vehicle from a standing start. Also, a fluid coupling device may be installed between the engine and the belt-type CVT to ensure starting acceleration and, if a torque converter is utilized to increase starting torque.
Secondly, belt-type CVTs have no reverse output capability. Thus, a compound planetary gear set with a ring gear brake is often installed between the engine and the CVT to reverse the input rotation direction to the CVT, thus reversing the CVT output rotation direction. Furthermore, belt-type CVTs generally have a high output speed ratio capability. For example, the output speed ratio of belt-type CVTs can range approximately from 1:0.4 to 1:2.5. However, the maximum speed ratio of transmission output for automobiles rarely exceeds 1:1.5, which is a 0.66:1 overdrive speed ratio. In order to both accommodate and utilize the full output speed ratio potential of belt-type CVTs, an additional speed reduction needs to be applied to the transmission output or to the vehicle final drive for optimum performance.